Electrically powered vehicles which obtain a vehicle driving force from a motor, such as electric vehicles or hybrid electric vehicles, have a secondary battery mounted therein, and the motor is driven by electric power stored in the secondary battery. Electrically powered vehicles have the function of regenerative braking, that is, braking in which the motor is caused to function as a generator during vehicle braking to convert the kinetic energy of the vehicle to electric energy. The electric energy thus obtained is returned to the secondary battery, and reused during, for example, acceleration.
Because overdischarge or overcharge of a secondary battery will deteriorate the performance of the battery, it is necessary to control charging or discharging while keeping track of the state of charge (SOC) of the secondary battery. For example, in a hybrid electric vehicle, in order that the secondary battery may accommodate regenerative power, or can immediately supply power to the motor when a request is received, the state of charge is controlled to approximately the midpoint (SOC=50% to 60%) between a fully charged state (SOC=100%) and a completely discharged state (SOC=0%). Therefore, it is necessary to accurately detect the SOC of the secondary battery, and to this end, it is necessary to precisely detect the voltage of the secondary battery.
The voltage of the secondary battery can be detected using a flying capacitor. Specifically, a flying capacitor is connected to both ends of the secondary battery with an input side sampling switch being interposed therebetween, and a differential amplifier circuit is connected to the flying capacitor with an output side sampling switch being interposed therebetween. First, the input side sampling switch is turned ON to hold the voltage of the secondary battery in the flying capacitor. Then, the input side sampling switch is turned OFF, and the output side sampling switch is turned ON to supply the voltage stored in the flying capacitor to a non-inverting input terminal and an inverting input terminal of the differential amplifier circuit so that a potential difference between the two input terminals is detected in the differential amplifier circuit to thereby detect the voltage of the flying capacitor, that is, the voltage of the secondary battery. Specifically, an output voltage output from the differential amplifier circuit is supplied to an arithmetic circuit or a CPU (a microprocessor), and the microprocessor reads the output voltage.